This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Katekyo Hitman Reborn!
Gokudera's Varia fight for the Storm Ring. The end where Gokudera sacrifices the Ring he's been beaten near-death for, because he wants to survive another day for his friends, "I wanted to see the fireworks again, so I came back..."
When Tsuna arrives in the Future in his own coffin and those pained faces Gokudera makes.
The fight with Gamma. Gokudera's line after Gamma reveals that the Tenth Vongola Boss was gunned down in plain view of many of his subordinates gets me teary every time, especially since you know it's gonna make the poor boy dive into a fight he definitely won't win. Made even worse by the fact that when he loses, Gamma tortures him for information and Gokudera responds by spitting in his face.
Even after returning from the future, just mentioning Yuni can still bring tears.
Then there is the Cloud fight in the Ring battles. Kyoya beats Gola Mosca easily with one hit despite the creature getting up again.But then we find out the truth: Xanxus put his own adoptive father in Gola Mosca as his POWER SOURCE. When Tsuna shot down Gola Mosca when it got back up, he effectively attacked the ninth boss and (nearly) killed him. The ninth boss who is described as a nice old man, who looks like a nice old man, who IS a nice old man. Tsuna just attacked him.And the fact that this shocks Tsuna out of Hyper Mode, causing him to blame himself until the dying old man tells him that it isn't, that it's his(the ninth's) own weakness that caused all this and after telling them what really happened to Xanxus during the Cradle Affair, he then tells Tsuna about how Reborn tells him all about Tsuna's life and the girl he likes and everything. He admits that Tsuna's a child who isn't suited for being a mafia boss with his soft heart but that's EXACTLY why Tsuna was chosen. At this point he has touched Tsuna's forehead. This is when the real Tearjerking occurs.
Anyone who's lost someone they truly loved would understand how Daemon felt when his lover Elena was killed.
Yuni with tears in her eyes in Chapter 377: "My heart would be in great pain, as if it's being gouged out."
In Chapter 388 Tsuna acts quite upset when he learns that Reborn will either die or become a Vindice member.